In conventional paper dryers of the prior art, respective upper and lower felts are guided around upper and lower dryer rolls by felt rolls. In the course of a drying operation on such a conventional dryer unit, the paper web to be dried is sandwiched between the felts and the dryer rolls but is unsupported in the region in which it passes from a lower dryer roll to an upper dryer roll and vice versa. While such a dryer unit provides an efficient drying, it is not capable of as high speed operation as is desirable owing to the presence of the "draws"or regions in which the paper web is unsupported as it passes between the upper and the lower dryer rolls.
Recently there have come into use "unifelt" or "serpentine" dryer units in which a single endless felt is trained around upper and lower dryer rolls and associated guide rolls. Dryer sections of this type are capable of operating at speeds in excess of 200 feet a minute owing to the fact that the paper web is supported by the felt or fabric over substantially the entire extent of the dryer.
While the "unifelt" dryer units of the type described hereinabove are capable of high speed operation, they incorporate a number of defects. One of these defects is a buildup of air pressure at the nip between the felt and a dryer roll at a location at which the felt is moving toward the roll. Owing to the high surface velocity of the fabric or felt as it approaches this nip, it entrains a relatively large amount of air and causes a buildup of pressure at the nip. At a location on the dryer at which the paper web is relatively below the fabric at such a nip, the buildup of pressure causes a bubble to develop between the fabric and the paper web. At a later point in the drying operation, such a bubble results in a crease or overlapping which produces a defect.
Attempts have been made in the prior art to solve the problem outlined hereinabove of pressure buildup at the nip between the fabric and the dryer roll. Such efforts have not been as successful as is desirable. They may require relatively expensive and cumbersome ducting to reach the points at which the pressure buildups occur. They involve the danger that loosened fasteners or the like may fall into the nip with the consequent damage, not only to the paper web being formed, but also to the expensive machine fabric or felt.